1. Field of the Invention
This invention relates to a super-telephoto zoom lens suitable as an interchangeable lens for a single-lens reflex camera, an electronic still camera or the like.
2. Related Background Art
Usually, it is often the case that a super-telephoto lens photographs distant objects, and once the camera is installed, it is impossible to simply change the photographing distance as when carrying about a medium-telephoto lens or a wide angle lens and interchanging it. Thus, a super-telephoto zoom lens which can select angles of view from telephoto to super-telephoto in a moment with a camera fixed at a certain location compares with one photo-taking lens which yet provides several telephoto or super-telephoto lenses prepared.
Now, in most focusing systems for conventional popular zoom lenses, a lens unit which is disposed forwardly of a vari-focusing portion and most adjacent to the object side is moved toward the object side along the optical axis thereof to thereby effect focusing and therefore, particularly a telephoto zoom lens in which even at the wide angle end, the focal length of the whole system is very long has the tendency that the focal length of a first lens unit becomes long, and the amount of movement for focusing becomes great, and this has led to bulky and complicated structure. In addition, it is difficult to use a zoom lens including a telephoto area and in which the position of the centroid fluctuates greatly during vari-focusing. Also, conventional zoom lenses have suffered from the disadvantage that their brightness is dark as compared with that of single-piece telephoto lenses. So, applicant proposed in Japanese Laid-Open Patent Application No. 59-23314 an internal focus telephoto zoom lens as a telephoto zoom lens which eliminates these disadvantages.
However, in the internal focus telephoto zoom lens disclosed in Japanese Laid-Open Patent Application No. 59-23314, the focal length at the telephoto end is of the order of 800 mm and the angle of view has not been sufficiently small. Also, most telephoto lenses of which the focal length exceeds 800 mm are single-focus lenses and it is impossible for them to move in a-moment to effect predetermined framing, and there has been no way but to substitute the trimming during enlargement for such framing.